Digital electronic systems typically utilize very large scale integrated circuit ("VLSI") blocks. VLSI blocks, are interconnected to each other within an electronic system by electronic conductors that act as transmission lines. Digital HI and LO signals are represented by two distinct voltage levels presented on the conductors. A "driver" forming part of one VLSI block couples a transmitting VLSI block to the transmission lines. Similarly, a "receiver" forming part of a receiving VLSI block, couples the receiving VLSI block to the lines. Each driver and receiver typically comprises a plurality of transistors formed on the respective VLSI blocks.
The transistors forming the drivers and receivers are coupled to ground points on the VLSI blocks. These ground points, however, are not at the same potential as the external ground potential of the VLSI packages. Drivers are coupled to the external ground connection through a package impedance. This package impedance typically has resistive, inductive, and capacitive components and is therefore a source of electrical noise present when a driver output switches from LO to HI or HI to LO. This noise is typically referred to as a "ground bounce".
At the receiver, binary HI and LO signals are typically distinguished by their voltage levels relative to the receiver package ground. Signals that exceed a threshold voltage level represent a digital HI while signals that fall beneath another threshold voltage represent a digital LO. As will be appreciated, the presence of noise from the transitioning signal at the transmitter, may cause a signal not intended to cross a threshold to cross this threshold, as sensed at a receiver. This, in turn, may lead to errors in the received signal.
Numerous digital drivers and receivers address this problem. For example, the gunning transceiver logic ("GTL") family as more particularly described in U.S. Pat. No. 5,023,488 uses low voltage swings that reduce transient effects of parasitic impedances, including package impedances. As well, this patent discloses clamping the drain to source of a GTL driver to reduce the rate at which current is drawn so as to provide increased damping for noise due to transient voltages.
Still other logic families use differential outputs. As the effect of parasitic impedances is the same for both outputs of a differential pair, differential signals naturally reject common mode noise. Differential signals, however, require double the number of outputs for a transmitting VLSI block; double the number of transmission lines interconnecting the transmitting VLSI block to the receiving VLSI block; and double the number of inputs at the receiving VLSI block. As will be appreciated, it would often be desirable to eliminate these extra inputs, outputs and interconnects.
Accordingly, an alternative to known approaches to reduce the effects of package impedance in digital transmitters and receivers is desirable.